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Mitochondria isolated from sugar beet ( Beta vulgaris L.) taproot were shown to swell spontaneously after the transfer from a sucrose-containing isolation medium to isoosmotic potassium chloride solutions. The kinetics of this process was strongly retarded after the replacement of potassium with sodium in the incubation medium and was substantially stimulated by the electron-transport chain activity and valinomycin. At neutral pH of the incubation medium, the rate of K + -dependent swelling of mitochondria decreased by 30-50% after adding 1 mM ATP but was insensitive to other nucleotides (GTP, UTP, and CTP). In the medium acidified to pH 6.0, the addition of ATP caused shrinkage of mitochondria that had been swollen in the KCl medium. In the absence of this nucleotide, the kinetics of K + -dependent swelling of mitochondria was considerably decelerated upon the acidification of the incubation medium. The effects of ATP were independent of the presence or absence of oligomycin and atractyloside. However, the ATP-dependent shrinkage of mitochondria was inhibited in the presence of quinine, and this agent also inhibited K + -dependent swelling of organelles in potassium acetate solutions. The presence of K + ions in the incubation medium caused a rapid dissipation of the mitochondrial membrane potential ( ∆ψ ) that was generated during succinate oxidation. The addition of ATP to the reaction medium resulted in the oligomycin-insensitive restoration of ∆ψ . The results are regarded as evidence that the membrane of taproot mitochondria is endowed with functionally active ATP-sensitive K + uniporter. This system is likely to represent a K + channel that catalyzes the electrogenic transfer of potassium ions to the mitochondrial matrix. It is supposed that the membrane of taproot mitochondria also contains a quinine-sensitive K + /H + antiporter that catalyzes the efflux of potassium from the matrix or, on the contrary, the accumulation of K + in the presence of potassium acetate.